"These girls have become Muslims," Shekau said in the 17-minute recording. "We will never release them until after you release our brethren."

In one part of the recording, the girls recite “Al-Fatiha” -- the first chapter of the Quran -- in Arabic. "Al-Fatiha," which comprises seven verses, is one of the first prayers taught to those learning Islam.

AFP declined to release a word-for-word translation of the girls' comments because they were being held as hostages. It also reported that Nigeria's interior minister Abba Moro quickly rejected the idea of a prisoner release.

International outrage has grown about the fate of the girls, who were seized on April 14 when militants from Boko Haram stormed a secondary school in the northeastern village of Chibok. The militants took 276 girls who were taking exams. Some managed to escape but around 200 remain missing.

"My sense is that President Goodluck Jonathan is not someone who is interested in negotiation with Boko Haram. Given the level of aggression that the Nigerian government has used when countering Boko Haram I don't think any [prisoner] release is forthcoming."

He said the sign that many of the girls were in one group was welcome – but did not mean a rescue attempt was any closer.

"A rescue is always going to be very, very difficult," Leiter added. "This is an extremely remote area and the Nigerians have not in the past taken a lot of assistance from the United States so the fact they are in one group makes it more possible but I wouldn't underplay how hard it would be for either the Nigerians alone or even with U.S. assistance."